Revenue - Disaggregation of Revenue (Details) - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2019
Disaggregation of Revenue [Line Items]
Revenues	$ 17,729	$ 19,577	$ 20,753
United States
Disaggregation of Revenue [Line Items]
Revenues	5,983	7,225	7,677
U.K.
Disaggregation of Revenue [Line Items]
Revenues	2,413	2,776	3,175
Other Europe
Disaggregation of Revenue [Line Items]
Revenues	5,129	5,121	5,294
Australia
Disaggregation of Revenue [Line Items]
Revenues	1,529	1,487	1,582
Other International
Disaggregation of Revenue [Line Items]
Revenues	$ 2,675	$ 2,968	$ 3,025